revset: added lazyset implementation to closed revset
revset: added lazyset implementation to converted revset
revset: added lazyset implementation to extra revset
revset: added spanset class to represent revision ranges
revset: added basic operations to spanset

Added methods __add__, __sub__ and __and__ to duck type more methods in
baseset
revset: added operations to spanset to duck type baseset

Added more operations which are not lazy but only used so far to duck type
baseset.